JSFiddle - React, Tailwind, and code Playground

HTML

<form name="priceCalc" action="">
    <div align="center">
        <select name="squareOrBanner" onchange="price();">
            <option value="1">Square</option>
            <option value="2">Banner</option>
        </select>
        
        <select name="number" onchange="price();">
            <option value="250">250</option>
            <option value="500">500</option>
            <option value="750">750</option>
            <option value="1000">1000</option>
        </select>
        <br>
        <div id="prices">Change items to see price</div>
    </div>
</form>

JavaScript

function price() {
    
    var sqBn = document.priceCalc.squareOrBanner;
    var Amt = document.priceCalc.number;
    
    var price = parseInt(sqBn.value) * parseInt(Amt.value) * 0.2;
    
    document.getElementById("prices").innerHTML = 'Price: ' + price;
}